You can use PowerTunes to create multiple libraries. Perhaps you and your spouse have very different tastes in music, as well as separate iPods. Hey, I can relate. My wife's Paul Simon and 10,000 Maniacs collections keep butting heads with my Marilyn Manson tracks. Can't we all just get along?
Perhaps you'd like to keep a library for different settings; one for home and one for work. PowerTunes makes it easy. That the main interface makes the iTunes quite a bit larger, so understand that it will consume a lot of screen space when running.
Other spiffy features include an option to merge libraries, move your library off your main hard drive and cleanup orphaned tracks.
It will cost you $19.95US for a single license. Sure, you can do all of these tasks manually (read: free), but the convenience of "click and it's done" is nice. Plus, many users (not the geeks like you and me) couldn't be bothered with the manual method. If that's you, check this out. It's a nice piece of software.